The [5,10,15,20-tetrakis((pentaammineruthenio(II))-4-pyridylporphinato]cobalt(II) complex that catalyzes the four-electron reduction of O-2 at graphite electrode surfaces on which it has been assembled has now been prepared in homogeneous solution. The preparative procedure for the catalyst is described, and the kinetics of its catalysis of the reduction of O-2 by chemical reductants in homogeneous solution are reported. In solution, only two-electron reductions of O-2 are obtained and the rate of intramolecular electron-transfer from the four Ru(NH3)(5)-py(2+) centers on the periphery of the catalyst molecule to an O-2 molecule coordinated to the Co(II) center in the porphyrin ring is shown to be quite slow. Possible reasons for the contrasting mechanistic behavior of the catalyst in solution and on graphite electrodes are suggested.